1. Field of the Invention
The invention relates to power input controls for AC induction motors, and more particularly to a control circuit for providing variable input power in accordance with loading on the motor.
2. Description of the Prior Art
AC induction motors are well known and widely used. These types of motors run at substantially constant speeds independent of load and applied voltage. The most efficient operation results when the applied voltage is a function of the load, since the motor draws substantially the same current whether loaded or unloaded.
Electrical power is wasted when the motor is partially loaded or is unloaded because relatively large currents flow and little work is performed. IR losses occur in the distribution system, including the motor windings, even though no mechanical power is delivered.
Motor control systems are known which reduce power losses by sensing the phase lag between voltage and current. The control system forces the motor to run at a constant predetermined power factor regardless of load or line voltage variations, within the limits of the motor.
An example of a control system is shown in Nola U.S. Pat. No. 4,052,648, and NASA Tech. Brief, MFF-23280, summer 1977. Voltage is varied by a selectively triggered power switch. A phase control and firing angle circuit is provided, including a ramp voltage wave form synchronized with the zero crossings of the sinusoidal load voltage, a DC error signal, and a train of pulses that become wider as the error signal increases. The error signal is derived from a voltage/current phase lag sensing circuit which produces a DC voltage proportional to the phase lag. Since the current remains high in an unloaded motor, the phase angle between voltage and current shifts with load. Typically, the current may lag the voltage by 80.degree. in an unloaded motor and by only 30.degree. when loaded. The produced DC voltage proportional to phase lag is summed with a fixed reference voltage which is indicative of a desired phase angle. The difference of these two is the error signal which biases the ramp voltage which is in synchronization with the line voltage. The intersection of the ramp and error voltages are detected by a squaring amplifier whose output provides the timing for turning on the power switch. The ON time of the power switch thus varies with the load. This in turn varies the voltage applied to the motor, to force the phase angle to remain at the commanded value. The phase angle is measured by detecting the time between the zero crossings of the voltage and the zero value of the trailing edge of the current.
While prior power factor controllers have been useful for their intended purposes, the present invention relates to improvements thereover.